MLK-13508-2 mx6sllarm2: Update DDR script to v2.3

DDR scripts are updated to fix DQS gating issue commonly for LPDDR2 and
LPDDR3. That DQS sampling may have problem after enabling the
SDE_0/SDE_1 in MDCTL.

Changes:
  -Based on V2.2, move the "Read DQS Gating Disable" to the step after
   "MR setting", to avoid potential DDR initializaiton failures
   (especially in Plugin Mode).

File:
  http://compass.freescale.net/livelink/livelink?func=ll&objid=235701297&objAction=browse&sort=name&viewType=1



Test:
  Passed stress test on 1 LPDDR2 ARM2 board and 1 LPDDR3 ARM2 board.

Signed-off-by: Ye Li <ye.li@nxp.com>